(1) Field of the Invention
This invention relates to a recording method in magnetic picture recording systems which records video and audio signals in frequency-division multiplexing. This invention, more particularly, relates to:
(a) a recording method which can offer high quality pictures free of color phase irregularities and which is less susceptible to crosstalk in audio signals and to the influence of audio signals on pictures, even when color video and audio signals are recorded in high-density and in multiplex without guard bands;
(b) a method to conduct track servo-control by means of audio signals;
(c) a recording method for field skip recording wherein the audio signals of the skipped field are recorded in a form to improve the efficiency of the recording regions of a magnetic disk and the recorded signals are used to reproduce continuous audio voice.
(2) Description of the Prior Art
Two systems to improve recording density for recording video signals on a magnetic tape or a magnetic disk are known as a phase modulation system (PM) and a frequency modulation system (FM) by tilted azimuth.
In the FM method using tilted azimuth, as is well known in the art, luminance signals which have been frequency modulated or FM luminance signals are recorded at different azimuth between adjacent tracks. Even if a video head overrides the adjacent tracks at the time of reproducing, signals from the tracks of different azimuth will not mix except for the low frequency components. Therefore, even though the signals are recorded in a high density without guard bands or partially overwritten and a magnetic head is placed overriding the adjacent tracks for reproducing, crosstalk will not occur. Such azimuth effect can not be expected in the case of color video signals because a low frequency band is assigned to color signals for recording. It is, therefore, general practice either to record by rotating the phase of the so-called low band conversion color signals which are obtained by lowbands converting the carrier color signals by 90.degree. per one horizontal scanning and at the same time inverting the phase per one track (PS system) or to record by inverting the phase of color signals for one horizontal scanning in one track but by recording with a fixed phase on the track adjacent thereto (PI system). Crosstalk can be reduced by interleaving the frequencies between adjacent tracks.
As disclosed in Japanese Patent Publication No. 56-51406 and Patent Application Laid-open Print No. 53-41126, the PM recording system is characterized as follows:
(1) the carrier to be modulated by luminance signals is synchronized with the relative movement between a video head and a magnetic medium;
(2) the modulation index mp is controlled to be 1.3 radian or less when the carrier is PM modulated by the luminance signals;
(3) these modulated luminance signals or carrier luminance signals are recorded in such a way that the positions of vertical and horizontal synchronizing signals in the adjacent tracks become aligned with each other and the carrier phases in adjacent tracks become also aligned.
According to this system,
(i) the amplitudes of carrier components recorded on respective tracks are substantially constant because mp.ltoreq.1.3 and the carrier components in the reproduced signals become constant even though the magnetic head overrides the adjacent tracks because the components are of the same phase as the tracks,
(ii) because mp.ltoreq.1.3, components of more than a secondary side band may be disregarded and because synchronous signals are aligned between tracks, the side band components of the reproduced signals merely become the composite of plural frames with a high correlation even if the video head overrides the adjacent tracks,
(iii) therefore, luminance signals can be demodulated free of crosstalk even if they are recorded without guard bands or overwritten partially or even if tracking errors occur in reproduction.
In the case of color video signals, as color signals are recorded, too, an auxiliary carrier is phase-modulated with the color signals under the same conditions as the luminance signals, or the auxiliary carrier is suppressed-carrier modulated with the color signals so that the phases of carriers become aligned between the tracks in recording. This overcomes crosstalk. Alternatively, the phase of the color signals is rotated by 90.degree. per one horizontal scanning and inverted per one track in recording (PS system) or the phase of the color signals is inverted for a track for one horizontal scanning but fixed for the track adjacent to the above (PI system) just like in the tilted azimuth recording method. This cancels crosstalk.
The system of high density magnetic picture recording has been improved as mentioned above, but the audio signals which should accompany the video signals have been left undeveloped except for the tilted azimuth recording method not beyond the level that audio signals are recorded in separate tracks. This imposes a limitation on the general efficiency of a magnetic medium as well as requires an additional head for audio signals. In order to solve such problems, audio signals may be recorded together with video signals in frequency multiplex. Although such a method causes no problem in case video signals are recorded with guard bands, as there is no correlation between audio signals of adjacent tracks in the case of recording without guard bands, crosstalk among audio signals becomes unavoidable by such simple multiplex recording and hence can not be put into practice.
On the other hand, there has long been desired an improvement for sound quality in a magnetic recording system. Bias recording system using a fixed head similar to the audio tape recorder has conventionally been used for recording/reproducing audio signals in VTR of the tilted azimuth FM system, recording frequency characteristics and wow-flutter which are dependent on the running speed of the magnetic medium such as a tape are increasingly deteriorated as color picture images are more and more recorded in a higher density for a longer time.
Many efforts have been made to improve heads and voice circuits (such as a noise-reduction circuit) in order to better the sound quality. In recent years, a FM modulation recording method for audio signals using a high-speed rotating head which is unique to VTR was developed in the field of household VTRs, and a Hi-Fi video system was made public in the magazine The TV Gijutsu, July 1983, aiming at sound quality as high as that in PCM (pulse code modulation) recording.
The proposed systems of this type can be classified roughly into two:
In (a) one of type, FM audio signals of two channels for stereo recording are formed with a suitable deviation in a manner to position at substantially the lower side of the FM luminance signal band zone in frequency allocations of VHS video recording signals, i.e. low band conversion color signals and FM luminance signals respectively, and the FM audio signals are recorded with a special rotating head for audio signals on a tape, and then the FM luminance signals and low band conversion color signals are recorded with a video rotating head in a conventional manner.
Audio and video signals are sequentially recorded twice in multiplex but they are taken out separately for reproduction. More specifically, by using a special compound of four heads comprising two video heads and two audio heads which are provided 120.degree. ahead of the video heads, FM audio signals are first recorded with the audio heads positioned ahead on a tape and then the video signals are recorded with the video heads. The audio signals which have been recorded before are therefore erased on one surface and are recorded in a deep layer.
The audio signals reproduced from such recording tape interfere less with video signals, although they are somewhat lower in level. This method (referred to as VHS system hereinafter) can therefore be regarded as one which effectively utilizes the merits of FM modulation.
(b) Another type is the system wherein a space is made between the frequency band occupied by the color signals which are low-band converted and the frequency band occupied by the luminance signals which are FM modulated and FM audio signals are inserted into the interval space for recording. More particularly, FM audio signals are multiplexed with the video recording signals which have conventionally been used for VTR to be recorded with a conventional rotating video head of the two-head type. The system is advantageous in that voice can be recorded at high fidelity without the necessity of modifying the conventional recording system used for VTR. (This system is referred to as a Beta system hereinafter).
It is critical to remove crosstalk between audio signals in reproduction and the interference from the audio signals to the video signals in both systems. In the VHS system, azimuth recording is adopted by setting an azimuth angle in respective rotating audio heads in order to remove crosstalk between the audio signals of adjacent tracks.
In the Beta system, on the other hand, one pair or left and right carrier frequencies of FM modulation are switched to each of the recording tracks which are adjacent to A and B tracks for recording. In other words, carriers of four different frequencies, i.e. left and right signals of audio signals and tracks, A and B, are used for recording to obviate the problems. In the Beta system which does not adopt the deep layer recording system like the VHS system, the level of audio FM carriers is made lower than that of the FM luminance signals and the audio carrier frequencies are made to interleave with those of video signals in the determination of FM carrier frequencies in order to prevent mutual interference between video signals and audio signals.
As described above, due to the development achieved in multiplex recording technology of audio and video signals, the video recording at higher sound quality for a longer time recently became possible in the FM system by means of the titled azimuth technique.
In the VHS system, as audio signals are first recorded by a separate special audio head with azimuth and then video signals are recorded by a conventional video head, the interference between the video and the audio signals or the crosstalk among audio signals are almost completely avoided. But a question may be raised whether the audio head which is additionally provided is really worthy or not when the system is judged as a whole. The Beta system is not entirely free from problems such that the FM luminance band width has to be narrowed compared to the conventional Beta system in order to secure the FM audio frequency bands which can separately reproduce at sufficient S/N ratio and that complicated measures should be taken for the level variation of the color video signals caused by the approach of the audio signal band to the auxiliary carrier band of the low conversion color signals, even though the system is convenient in that a conventional video signal head can double as the audio head.
In the case of magnetic recording by field skip, there arises a problem of how to deal with the voice of the skipped fields.
As is well known in the art, the field skip recording method is effective in the case where almost similar video picture images continue. In this method, such similar video picture images are culled in recording to the extent not to affect the picture quality, and at the time of reproducing the recorded track is repeatedly reproduced for the number equal to the skipped fields. Although the movements in pictures become unnatural by this method, the method can remarkably improve the recording density in a broader sense as it can save recording time even if the recording medium and the density are the same. Another advantage of this method is that it can alleviate the restriction in the control mechanism that a recording head should be fed in an extremely short time when transferring from the end of one track to the beginning of the next track. This restriction is especially acute in concentric disk recording. For instance, when one field of video signals of NTSC system is recorded in one rotation of a disk without field skip, it is necessary to control the feed of the head so that it is fed once every 1/60second but it should be completed in its vertical retrace period in about 200 .mu. seconds. But when field skip is conducted for every one field, the head may be slowly fed once every 1/60 second, which corresponds to one field, thereby simplifying the head control mechanism remarkably. However, recorded data becomes inevitably sporadic timewise.